Conventionally, a heating equipment of this type utilizes radiant heat from flames obtained by combustion in one time and sensible heat of a combustion gas at a high temperature.
In such a conventional heating equipment, it raises the combustion temperature too high to control the temperature within permissible skin temperature of a heating pipe to utilize most of oxygen in the air for combustion. Thus, it has been impossible to suppress generation of pollution materials, viz., NO.sub.x and uncombustibles.
Thus, there has been no other method in a conventional combustion equipment but to elongate the length of combustion flames so as to bring the temperature of a combustion gas down to a low temperature.
Besides, there has been such a drawback that a heat exchanger becomes large in size when heat exchange is effected between combustion exhaust gas and blown air.
In case multi-stage contact combustion is utilized, there have been such drawbacks that a heating equipment has to be provided in each number of combustion times, that cost of the equipment becomes expensive, and that heat loss is increased because a surface area of the equipment is increased, thus degrading thermal efficiency.